Credit: Courtesy of Provasoli-Guillard National Center for Culture of Marine Phytoplankton

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In less than one billionth of a second, plants from algae to redwoods transform 95 percent of the sunlight that falls on them—1017 joules per second bathe the planet—into energy stored chemically as carbohydrates. The quantum key to doing that lies in a phenomenon known to physicists as quantum coherence, according to new research published in Nature on February 4. (Scientific American is part of Nature Publishing Group.)

Quantum coherence describes how more than one molecule interacts with the same energy from one incoming photon at the same time. In essence, rather than the energy from a particular photon choosing one route to pass through the photosynthetic system, it travels through multiple channels simultaneously, allowing it to pick the quickest route. "The energy of the absorbed light is finding more than one pathway to move along at any one time," explains physical chemist Greg Scholes of the University of Toronto, leader of the research group that highlighted the effect. "We can't pinpoint the energy of that light. It's shared in a very special way."

Scholes and his colleagues isolated the "antenna" (a protein chain that propagates the incoming energy) of photosynthetic organisms known as cryptophytes, specifically marine algae Rhodomonas CS24 and Chroomonas CCMP270. Cryptophytes are special because they do not all employ the same protein to harvest the energy in sunlight, like the chlorophyll ubiquitous in green plants. "These guys customize their antenna protein," Scholes says, noting that the algae also have flagella that permit them to move around. "They're quite different colors."

The algae's different antenna colors allowed the chemists to pulse the specific proteins with femtosecond (one quadrillionth of a second) bursts of laser light. Based on atomic scale maps provided by previous X-ray crystallography, the researchers tracked the energy as it entered the photosynthetic system and progressed through it to so-called reaction centers, where the energy storage occurs. The pulses revealed that within single protein molecules the energy traveled down multiple pathways simultaneously. Thus, the protein antennae's efficiency relies on quantum coherence, such that molecules within a protein separated by vast distances (at the atomic scale) acted in a similar fashion at the same time for a relatively long period of time—more than 400 femtoseconds.

Whereas previous research had shown that purple bacteria used quantum methods to efficiently harness light—and prior experiments had shown similar quantum effects in green sulfur bacterium that had been cooled to 77 kelvins (–196 degrees Celsius)—this experiment was the first conducted at room temperature, 294 K, to replicate such effects. Basically, according to this research, an incoming photon created a series of ripples, like a stone thrown into a pond, that interfere with each other to allow the energy wave to explore all potential pathways through a given protein molecule at the same time, allowing no energy to be lost to any wrong paths. It is as if you could drive to work via three different routes at the same time, losing no time or energy to traffic delays on any of the given routes, Scholes says. That allows the photon to travel to the reaction center almost instantaneously.

"In the systems we studied, even at room temperature, you can have these quantum effects and they're rather significant," Scholes notes, adding that means the effects are "biologically relevant" (used by the cryptophytes in their daily existence.). "The short laser pulse is used to expose the phenomenon, not to create it."

Chemist Graham Fleming at the University of California, Berkeley, has shown that such effects are visible in chlorophyll systems at low temperature. And biophysicist Gregory Engel of the University of Chicago, who was not involved in this research, argues that such effects are therefore likely to be used in all photosynthetic systems, allowing plants to efficiently transfer energy over long atomic distances. "That this effect appears in cryptophytes speaks to the generality of the process," Engel says. "This work will open the floodgates to new techniques to move and concentrate energy efficiently. It is extremely important for semiconductor devices [and] solar light harvesting."

In fact, such insights might help inform how to efficiently transfer energy over long atomic distances quickly in human-made systems to harvest sunlight—benefiting from nature's 2.7-billion-year head start in optimizing such systems. "Can it help you make a huge jump through space? It does precisely that," Scholes says. "It would be really nice to learn some tricks or what you need to think about if you want to design something that would move energy a long distance quickly."

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